Diesel Engine Fundamentals
DIESEL ENGINE SPEED,
FUEL CONTROLS, AND PROTECTION
Under these conditions, equal oil pressures are maintained on both sides of the buffer
piston and tension on the two buffer springs is equal. Also, the oil pressure is equal on
both sides of the receiving compensating land of the pilot valve plunger due to oil passing
through the compensating needle valve. Thus, the hydraulic system is in balance, and the
engine speed remains constant.
When the engine load increases, the engine starts to slow down in speed. The reduction
in engine speed will be sensed by the governor flyweights. The flyweights are forced
inward (by the spring), thus lowering the pilot valve plunger (again, due to the downward
spring force). Oil under pressure will be admitted under the servo-motor piston (topside
of the buffer piston) causing it to rise. This upward motion of the servo-motor piston will
be transmitted through the terminal lever to the fuel racks, thus increasing the amount of
fuel injected into the engine. The oil that forces the servo-motor piston upward also
forces the buffer piston upward because the oil pressure on each side of the piston is
unequal. This upward motion of the piston compresses the upper buffer spring and
relieves the pressure on the lower buffer spring.
The oil cavities above and below the buffer piston are common to the receiving
compensating land on the pilot valve plunger. Because the higher pressure is below the
compensating land, the pilot valve plunger is forced upward, recentering the flyweights
and causing the control land of the pilot valve to close off the regulating port. Thus, the
upward movement of the servo-motor piston stops when it has moved far enough to make
the necessary fuel correction.
Oil passing through the compensating needle valve slowly equalizes the pressures above
and below the buffer piston, thus allowing the buffer piston to return to the center
position, which in turn equalizes the pressure above and below the receiving
compensating land. The pilot valve plunger then moves to its central position and the
engine speed returns to its original setting because there is no longer any excessive
outward force on the flyweights.
The action of the flyweights and the hydraulic feedback mechanism produces stable
engine operation by permitting the governor to move instantaneously in response to the
load change and to make the necessary fuel adjustment to maintain the initial engine